It is known in general to remove volatile contaminants from soils, such as hydrocarbon saturated coil following an oil or petroleum spill, by heating the soil and driving off the contaminants through evaporation. Contaminated soil introduced into an elevated feeder end of a rotary dryer drum is mixed with heat supplied by a burner having a flame directed into the dryer. The heat drives the contaminants from the soil and into air drafted through the drum. The air is exhausted to a dust collector for cleaning before being released to a stack for venting to the atmosphere or to some other collection point. The cleaned soil is recovered at the discharge end of the drum.
The apparatus employed in such soil remediation process is not unlike that employed in a hot-mix process for coating aggregate with asphalt to produce highway paving materials and other similar bituminous mixtures. In the latter process, aggregate material stored in a feed bunker or bin is metered by gravitational or mechanical means at a controlled rate onto a conveyor belt or other transporter for delivery to an elevated end of an inclined rotary dryer drum, where it is dried and heated by the flame and hot gases produced by an oil or gas burner. After the aggregate is heated and coated with asphalt, the hot mix is recovered at the lower end of the drum, and discharged to a storage bin. Such hot-mix plant systems are described in the Asphalt Plant Manual, Manual Series No. 3 (4th ed. Dec. 1974) published by The Asphalt Institute, College Park, Md.
The conventional hot-mix asphalt plant dryer is a revolving cylinder, usually 3 to 10 feet in diameter and 15 to 40 feet long. It is equipped with longitudinal troughs or channels, called "flights," that lift the aggregate and drop it in uniform veils through the burner flame and hot gases. The length of time required for the aggregate to pass through the dryer is controlled by the slope of the cylinder, drum rotation speed, diameter, length, and arrangement and number of the flights. The dryer is normally designed to provide adequate heat and exhaust volume for a given input aggregate moisture content condition; and, the aggregate feed rate and fuel burn rate are appropriately adjusted to accommodate different moisture contents. For control of aggregate feed rate, provision is made to weigh the aggregate as it enters the dryer. To avoid overheating the aggregate during mixing, automatic control of burner temperature is provided in response to a temperature sensor installed within the aggregate stream in the dryer discharge path.
The hot-mix asphalt plant usually has one or more dust collectors for removing the dust generated during the mixing process from the exhausted air. Three commonly used types of dust collectors are mechanical collectors, fabric filters and wet scrubbers.
Mechanical collectors include skimmers and expansion chambers. These cause the settling out of heavier particles by decreasing the exhaust gas velocity. Mechanical collectors also include centrifugal ("cyclone") dust collectors which force the dust-laden air into a whirling motion to drive the dust particles outward by centrifugal force.
Fabric filters usually take the form of open-topped "bags" of felted, heat resistant fiber, held in cylindrical shape by internal frames, and vertically suspended within an airtight unit called a "baghouse." The baghouse is divided into a lower, dirty gas chamber and an upper clean gas chamber, with the bags hung so that exhaust air flows from the lower chamber from the outsides to the insides of the bags and into the upper chamber through the top openings. The size of the baghouse and number of filter bags used depends on the volume of exhaust air to be treated, the air-to-cloth ratio desired, and the cloth area per bag. A typical baghouse arrangement has several hundred bags, and includes a reverse-air jet or other mechanism to periodically remove dust cake buildup from the outsides of the bags. The baghouse dust collector is useful to remove the finer dust particles or "fines."
Wet scrubbers utilize water dispersed into droplets to entrap dust particles in the exhaust. Though wet scrubbers are efficient for cleaning the exhaust, they necessitate a source of water and also some means of disposal of the dust-laden water, which may itself become a pollutant.
The feeding, heating and dust collecting stages of known soil remediation systems, just as the corresponding stages of known hot-mix asphalt plant systems, comprise separate, isolated, often bulky components that must be individually transported to each new operation site and coupled together in a time-consuming assembly process, before soil contamination clean-up operations can begin.